The first thing to remember when trying to weather something: Less is more! Put on too much and it spoils the effect so be gentle. Like others have suggested cover it with a dull coat to take off the shine. I've used Rustall to good effect but I've found that a light dusting with a red primer will give a reasonably realistic appearance. Also it won't hurt to leave small areas shiny because it doesn't all rust at the same rate. I've included the link to my beater albums so you can get a variety of efforts. Some were more successful that others but you should be able to get an idea. The most important thing to do is practice on some scrap until you're comfortable with the technique.